1. Field of the Invention
The present invention relates generally to the field of bending boiler tubes for use in a furnace, among other things, and in particular to a new and useful method for rotary draw bending a boiler tube to permit tighter tube bends.
2. Description of the Related Art
Known methods for making tight radius 180 degree return bends (bends having a radius equal to the tube diameter, or 1.times.D) by rotary draw bending require the boiler tube wall thickness to be at least equal to almost 10% of the tube diameter. Tube clamps are used to clamp the tube and push longitudinally with a hydraulic cylinder to forcibly compress the tube. Simultaneously with the compression, the tube is bent in the desired radius. The compression-bending rotary draw method, also known as "boost bending", reduces the amount of tube wall thinning on the extrados of the bend from 30% without compression to about 15%-25%.
Other methods for reducing the amount of wall thinning in the tube extrados use gas or induction heat to heat the entire tube in the area of the bend prior to bending. These methods can only be used on bends greater than 1.times.D, and require a minimum wall thickness of 10% of the tube diameter.
Until now, methods for reducing the wall thickness of the tube without sacrificing tube strength of tightly bent tube walls due to wall thinning have not been available.